Oxidation of alkyl benzenes in the presence of alkanols



United States OXIDATION OF ALKYL BENZENES IN THE PRESENCE OF ALKANOLS NoDrawing. Application June 27, 1956 Serial No. 594,101

9 Claims. (Cl. 260-475) The present invention relates to the preparationof aromatic acid esters. More specifically, this invention relates to aone-step process for preparing esters of arcmatic acids from aromaticcompounds containing at least one alkyl group attached to the aromaticnucleus.

This application is a continuation-in-part of my copending applicationSerial No. 407,161, filed January 29, 1954, now abandoned.

In recent years, the esters of aromatic acids have become ofconsiderable importance, particularly those which find use asintermediates in the manufacture of high polymers of the filmandfiber-forming types. Extensive research has been conducted to providecommercially suitable processes for preparing the esters, and, at thepresent time, a large number of methods are detailed in the publishedliterature. In most processes, the ester is produced by first preparingthe aromatic acid from the corresponding alkyl benzene, and thereafteresterifying the acid by reaction with the corresponding alkanol.

The preparation of dimethyl terephthalate may be cited as typical. US.Patent 2,636,899, issued April 28, 1953, discloses the oxidation of adialkyl benzene (p-xylene) by dilute nitric acid at conditions of hightemperature and high pressure. The product obtained is of good qualityand can be esterified by the various methods known to the art to produceexcellent dimethyl terephthalate. However, the nitric acid consumedduring the atent O oxidation of the alkyl groups represents asubstantial proportion of the manufacturing cost of the'terephthalicacid produced. In addition, the investment and space required for thenecessary nitric acid-producing and -recovery equipment increases theover-all cost of a manufacturing plant.

The oxidation of alkyl benzenes by air has been investigated extensivelyand experimental studies have shown that air oxidation of one alkylgroup can be accomplished with ease at relatively mild reactionconditions. The oxidation of additional alkyl groups, however, presentsconsiderable difiiculty. For example, when p-xylene is air oxidized attemperatures of about 125 C., commercially feasible yields of p-toluicacid are produced. In order to air oxidize the p-toluic acid thus formedto terephthalic acid, temperatures of 200 C. and over and high pressuresare required to provide any substantial conversion. Due to theinsolubility of terephthalic acid in the reaction mixture and therequirement of intense agitation, a conversion of about 30% toterephthalic acid is the maximum which can be attained without thedevelopment of an unmanageable slurry. Both oxidations result in theformation of considerable quantities of unwanted residues consisting ofhigh-boiling complex material. In addition, operation at the highertemperature frequently results in the decomposition of i the organicreactant into carbon dioxide, which decomposition results, of course, inyield losses.

In US. Patent 2,653,165, issued September 22, 1953, the oxidation of themethylester of p-toluic acid is described. According to the processdescribed therein,

2,879,289 Patented Mar. 24, 1959 2 p-xylene may be oxidized first top-toluic acid by the conventional air-oxidation process, the p-toluicacid then is separated from the reaction mixture and esteriiied withmethanol at an elevated temperature, and the ester then is oxidized byair to methyl hydrogen terephthalate. The latter may be hydrolyzed toproduce terephthalic acid or esterified by methanol to produce dimethylterephthalate.

Although the foregoing process eliminates the requirement of a costlyoxidizing agent, the number of steps involved and the diflicultiesinvolved in the separation of the desired products from the undesirableresidues produced during the oxidation steps are such that the processis not completely satisfactory from a commercial viewpoint.

It is, accordingly, an object of the present invention to provide aprocess for the direct preparation of esters of aromatic acids. Afurther object is to provide a process for the direct preparation ofsuch esters in which air is used as the oxidizing means. Additionalobjects will become apparent as the present invention is more fullydescribed.

I have found that the foregoing objects may be attained when I provide aprocess which comprises subjecting an aromatic compound containing atleast one alkyl group attached to the aromatic nucleus to the combinedaction of an oxygen-containing gas and a lowermolecular-weight alkanolto produce in one step the cor- Example 1 Twelve hundred parts ofp-xylene, 7 parts of p-toluic acid, and 7.5 parts of a solution ofcobalt naphthenate (6% cobalt by weight) were charged into an autoclavefitted with a condenser leading to a decanter and a pressure-regulatingvent for release of noncondensable gases. The autoclave was pressurizedto 600 p.s.i.g. with air, and the charge was heated to 350 F. Air underpressure then was introduced under the surface of the liquid charge,which was being thoroughly agitated, at the rate of 1.5 standard cubicfeet per minute. When the temperature of the reaction mixture had risento about 375 F., methanol was introduced simultaneously with the airfeed for approximately 3 hours at a feed rate of about 25 parts perminute, a total of about 4400 parts of methanol being thus fed to thereaction zone, the temperature being held at about 375 F. throughout,and the'pressur'e maintained at 600 p.s.i.g. Approximately 4389 parts ofa mixture of p-xylene,'methanol, and water was collected in the decanterduring the run.

The product mixture (918 parts) was analyzed and found to contain 60% byweight of dimethyl terephthalate, 13.5% methyl p-toluate, 12.5%terephthalic acid, 4.0% methyl hydrogen terephthalate, and 2.2% p-toluicacid.

Example 2 Pres- Alco- Percent of Product Run Temp. sure hol 1 Time F.)(psi (parts! (hrs) g.) min.) DMT MP1 1 TPA 4 MHT 5 PTA A----- 375 600 253 60. 0 13. 6 12. 4. 0 2. 2 B 550 600 K 48 2 35. 5 35. 5 0.17 C-.. 500600 8 2 33.0 18. 1 i 30.6 D 475 250 5 32 2 50. 7 23. 8 3. 7 2.0 1. 4 E-475 600 a 48 2 15.0 56. 3 1. 5 F 475 600 17 1 12. 0 31. 4 16. 6 8. 0 20.8 G"... 475 600 5 17 2 41. 1 34. 8 2. 10 4. 3 2. 34 H 475 600 17 5 47. 121. 7 1.0

1 Methanol.

1 Dimet-hyl tetephthalate.

3 Methyl p-toluate.

4 Terephthalic acid.

5 Methyl hydrogen terephthalate.

6 p-Toluin acid.

1 p-Toluic acid omitted from initial charge.

5 Preheated methanol fed to the reactor.

' Total acids as PTA.

Example 3 20 Example 6 A mixture of 1200 parts of p-xylene and 9 partsof a cobalt naphthenate solution (6% cobalt) was charged to an autoclaveequipped as described in Example 1. After the charge was heated to 446F. and a pressure of 400 psi. ga. was attained, air at a rate of 1.5standard cubic feet per minute and methanol at a rate of 32 parts perminute were introduced into the charge, the air and methanol feeds beingcontinued for the duration of the run (5 hours). At the end of thistime, the reactor was cooled and the product mixture withdrawn. Theproduct mixture was analyzed and found to contain 83% dimethylterephthalate.

Example 4 A mixture of 400 parts of p-xylene and 2 parts of the cobaltnaphthenate catalyst was charged to an autoclave equipped with adecanter for removing the water of reaction. The autoclave waspressurized to 460 p.s .i. ga., and the charge was heated to atemperature of 428-465 F., the pressure and temperature being held atthese levels throughout the run. Then, air at a rate of 0.7 standardcubic foot per minute and ethanol at a rate of about 9 parts per minutewere introduced into the reactor, these feeds being continued for theduration of the run (1.75 hours). The total amount of ethanol fed was948 parts. After completion of the run, the autoclave was cooled to roomtemperature and then vented, and the reaction mixture was withdrawn fromthe autoclave. The mixture was filtered to separate solid material,which subsequently was extracted with aqueous sodium carbonate solutionto separate any acids present from solid esters. The decanter liquid andthe filtrate was distilled to remove ethanol and unreacted p-xylene, andthe residues were combined with p-xylene and extracted for 1 hour withaqueous sodium carbonate solution. The p-xylene layer was washed withsodium chloride and then distilled at atmospheric pressure to separatethe esters. Ethyl p-toluate and diethyl terephthalate were obtained in34% and about 3.4% yield, respectively.

Example 5 p-Xylene in the amount of 1200 parts and 8 parts of cobaltnaphthenate solution (6% cobalt) were added to the reactor described inExample 1. The reactor was pressurized to 400 p.s.i. ga. with air, andthen the charge was heated to 450 F. Isopropanol at a rate of 8 partsper minute and air at a rate of 1.5 standard cubic feet per minute werefed to the reactor for the duration of the run (3 hours), during whichtime the temperature was held at about 450 F. and the pressure at about400 p.s.i.ga. The total amount of isopropanol fed was about 1380 parts.At the end of the reaction period, the reactor wascooled to 100 F. andthen the product mixture was withdrawn. Analysis of thev mixture (1230parts) showed that the mixture contained 31% isopr pyl P- toluate and6%. diisopropyl terephthalate.

A mixture of 1200 parts of p-xylene and 7.5 parts of cobalt naphthenatesolution (6% cobalt) Was charged to the autoclave, which was pressurizedto 450 p.s.i.ga. with air. The charge was heated to 450 F. Air at a rateof 1.5 standard cubic feet per minute and l-propanol at a rate of 32parts per minute were introduced simultaneously into the heated chargethroughout the reaction period (4 hours). The temperature and pressurewere held at about the initial levels throughout the run. A total of.about 8060 parts of l-propanol was fed. At the end of the run, theautoclave was cooled to F., and the product mixture was removed andanalyzed. The mixture (1764 parts) was shown by analysis to contain 24%n-propyl p-toluate and 13% di-n-propyl terephthalate.

Example 7 A mixture of 1200 parts of durene and 7.5 parts of cobaltnaphthenate solution (6% cobalt) was charged to an autoclave equipped asdescribed in Example 1. The

autoclave was pressurized with air to 600 p.s.i.ga. and

the charge heated to about 500 F., the pressure and temperature beingheld at about these levels for the duration of the run. Methanol at arate of 32 parts per minute and air at a rate of 1.5 standard cubic feetper minute were introduced simultaneously into the heated mixture andfed for the duration of the run (6 hours). A total of about 10804 partsof methanol was fed. After completion of the run, the system was cooledslowly to 100 F., and the cooled product mixture was withdrawn from theautoclave. Analysis of the product mixture (1146 parts) showed that themixture contained 14% methyl durylate (17% yield), 17% of a mixture ofthe dimethyl esters of 2,5-dimethylterephthalic acid and 4,6-dimethylisophthalic acid (17% yield), 25% trimethyl methyltrimellitate(20% yield), and 37% tetramethyl pyromellitate (26% yield). A 5% yieldof acids (calculated as 2,5-dimethylterephthalic acid) was obtained.

Example 8 The procedure described in Example 7 was repeated, with theexception that the duration of the run was 3 hours, and the methanolfeed rate was 24 parts per minute. The total methanol fed was about 3205parts. The product mixture (1289 parts) was found, by analysis, tocontain 40% methyl durylate (42% yield), 38% of a mixture of dimethylesters of 2,5-dimethylterephthalic acid and 4,6-dimethylisophthalic acid(32% yield), and no trior tetraester. A 14% yield of acids (calculatedas 2,5-dimethylterephthalic acid) was obtained.

Example 9 Eight hundred parts of durene containing 5 parts of cobaltnaphthenate solution (6% cobalt) was charged to the autoclave, which waspressurized with air to 600 p.s.i.ga. After the charge was heated to 500F., the air and methanol feeds were introduced simultaneously into thecharge at rates of 1.5 standard cubic feet per minute and 32 parts perminute respectively. The temperature of about 500 F. and pressure ofabout 600 p.s.i.g. were maintained throughout the duration of the run (3hours). During this time a total of about 4773 parts of methanol wasfed. After the system was cooled to 100 F., the product mixture (796parts) was removed from the autoclave and analyzed. The analysis showedthat the mixture contained 22% methyl durylate (26% yield), 27% of amixture of the dimethyl esters of 2,5-dimethylterephthalic acid and4,6-dimethylisophthalic acid (25% yield), 13% trimethylmethyltrimellitate yield, and 24% tetramethyl pyromellitate (16% yield).A 10% yield of acids (calculated as 2,5-dimethylterephthalic acid) wasobtained.

Example 10 The procedure of Example 9 was repeated, with the exceptionthat the charge comprised 1200 parts of durene and 7.5 parts of cobaltnaphthenate solution (6% cobalt) and the air feed rate was 0.5 part perminute. The total amount of methanol fed was about 5583 parts. Theproduct mixture (1305 parts) contained, as shown by analysis, 25% methyldurylate (29% yield), 34% of a mixture of the dimethyl esters of2,5-dimethylterephthalic acid and 4,6-dimethylisophthalic acid (32%yield), 4% trimethyl methyltrimellitate (3% yield), and 9% tetramethylpyromellitate (6% yield). A 12% yield of acids (calculated as2,5-dimethylterephthalic acid) was obtained.

Example 11 The procedure of Example 9 again was repeated, with theexception that the charge comprised 1200 parts of durene and 7.5 partsof the cobalt catalyst, the reaction temperature was held at about 550F., and the reaction time was 2 hours. A total of about 4555 parts ofmethanol was fed to the reactor. The product mixture (1330 parts) thusobtained was determined by analysis to contain 43% methyl durylate (44%yield), 28% of a mixture of the dimethyl esters of2,5-dimethylterephthalic acid and 4,6-dimethylisophthalic acid (22%yield), 4% trimethyl methyltrimellitate (3% yield), and 11% tetramethylpyromellitate (6% yield). A 12% yield of acids (calculated as2,5-dimethylterephthalic acid) was obtained.

Example 12 In several runs, various hydrocarbons were subjected tosimultaneous treatment with air and methanol according to the procedureset forth in Example 4. A summary of the reaction conditions and theresults obtained in these runs is given in the following table:

air were introduced. At the end of the reaction period, the autoclavewas cooled to 100 F. and then vented. The product mixture was withdrawn.The mixture (916 parts) was shown by analysis to contain 78.3% dimethylterephthalate, 10% methyl p-toluate, and 2.4% acids (calculated asp-toluic acid).

Example 14 A heel consisting of 1500 parts of a p-xylene-methanolmixture was charged to a column reactor having pressure-controllingmeans and equipped for continuous operation. The reactor was pressurizedwith nitrogen to 385-400 'p.s.i.g., and the heel was heated to 590? P.Then, air at a rate of 0.3 standard cubic foot per minute and a 2/1 (bywt.) mixture of methyl p-toluate and methanol at a rate of about 9 partsper minute were introduced into the reactor. The methylp-toluatemethanol feed contained 100 p.p.m. of the cobalt naphthenatecatalyst (based upon the weight of the methyl p-toluate). For theduration of the run (ca. 29 hours), these feeds were continued and thetemperature and pressure were maintained at about the initial levels.The total retention time (based on holdup) was 4 hours. At thecompletion of the run, the feeds were stopped and the product receiverwas drained to a hold tank equipped with a condenser. Analysis of theproduct mixture showed the mixture to contain 44% dimethylterephthalate, 26% methyl p-toluate, and 16% acids (calculated asp-toluic acid).

Example 15 To the continuous reactor described in Example 14 was chargeda mixture of about 3200 parts of p-xylene containing 100 p.p.m. of thecobalt catalyst. After the mixture Was heated to 482 F. and a pressureof 1000 p.s.i.g. was attained, air at a rate of 7.0 standard cubic feetper minute was introduced into the reactor. After the reaction wasinitiated, a mixture of methanol and p-xylene in 4/1 mole ratio andcontaining 100 p.p.m. of the cobalt catalyst (based on the weight of thep-xylene) was fed to the reactor at a rate of 255 parts per minute. Theair feed and the p-xylene-methanol feed were continued for the durationof the run. Analysis of the liquid reaction mass indicated that 7.5% byweight of methanol was present in the liquid. After 4.75 hours, thereaction mixture was sampled, and the sample was analyzed and found tocontain 22% dimethyl terephthalate and 8% methyl p-toluate.

Example 16 About 4000 parts of methyl p-toluate containing 100 p.p.m. ofthe cobalt catalyst was charged to the continuous reactor. When thereaction temperature of 527 F.

. and the reaction pressure of 1000 p.s.i.g. were attained,

Cobalt Air Feed MeOH Total Percent Yield Run Naph- Temp. Rate Feed MeOHReaction No. Hydrocarbon (parts) thenate Range (std. Rate Fed T eCatalyst F.) c.f.m.) (parts/ (parts) (hrs.) Mono- Diester (parts) min.)ester A Toluene, 200 1 446-482 0.3-0.4 4 416 1.75 35 B Ethylbenzene,200". 1 464-518 0.3-0.4 4 408 1.75 39 C o-Xylene, 400 2 410-464 0.7-0.88 797 1. 75 25 8 D m-Xylene, 400 2 428-464 0.7-0.8 7 824 2 50 6 EOumene, 400 2 437-473 0.7-0.8 7 740 1.75 25 Example 13 the air feed tothe reactor was started. Upon initiation A mixture of 1200 parts ofmethyl p-toluate and 9 parts of cobalt naphthenate solution (6% cobalt)was charged to the autoclave, which then was pressurized to 600p.s.i.g.' The charge was heated to about 500 F., and then methanol andair were introduced simultaneously into the heated charge. Thetemperature and pressure were held at about the initial levels for theduration of the run (2 hours) during which time a total of about 1896parts of methanol and 300 standard cubic feet of Example 17 Theprocedure of Example 16 was repeated with the exception that thereaction temperature and pressure were 482 F. and 600 p.s.i.g., the airfeed rate was 3.4 standard cubic feet per minute, and themethanol-methyl p-toluate mixture (2/1 mole ratio) was fed to thereactor at the rate of about 164 parts per minute. The concentration ofmethanol in the liquid reaction mass was 0.6% by weight, and theretention time was 15 minutes. Analysis of a sample of the reactionmixture taken 35 hours after start of the organic feed showed that thedie methyl terephthalate content was 14.4%.

The foregoing examples illustrate the excellent results obtained byeffecting the formation of the carboalkoxy benzenes in one step insteadof in a number of steps as has been done previously. For instance, manyof the multistep processess of the prior art are characterized by theformation of substantial quantities of undesirable residues consistingof complex high-boiling materials. In contradistinction to many of thesemulti-step processes, minimum amounts of undesirable residues are formedin the one-step process of the present invention. Moreover, in carryingout the processes of the prior art, separate equipment is required forthe various steps, and additional equipment is required to effect theseparations necessary between the various steps. Following the procedureof the present invention, only one reactor is required and the reactionis carried out in a single step.

The process of the present invention provides superior thermalself-sufiiciency in that the requirement for heat removal associatedwith oxidation and heat supply associated with esterification whenoxidation and esterification are conducted separately is eliminated.Product handling is facilitated by the process of the present invention,because most esters are molten at the reaction conditions and theconcentrations of the insoluble acids are held at low values. Thus, inaddition to reduced equipment requirements, greater simplicity of designand operation is achieved by the present process. Moreover, the fluidityof the reaction mass increases the intimacy of contact of the reactantsto promote production of the desired products.

Furthermore, a significant manifestation of the fundamental advantagesinherent in the process of the present invention is found in theobservation that the addition of an alkanol to the reaction systemsimultaneously with the addition of an oxygen-containing gas effectsbeneficial results in that the amount of the alkyl benzene converted tocarbon dioxide is decreased. This manifestation may be seen by an.examination of the following data. In a series of runs conductedaccording to the procedure of Example 1, the rate of addition of thealkanol, in this case methanol, to the reaction zone was varied amongruns, Whereas all the other reaction conditions were held constant. Inall the runs, the initial charge comprised 1200 parts of p-xylenecontaining 9 parts of the cobalt catalyst and the operating conditionswere: temperature, 437 F.; pressure, 400 p.s.i.g.; air feed rate, 1.5standard cubic feet per minute; and time, 2 hours. The methanol feedrate and the yield loss incurred by the conversion of p-xylene to carbondioxide are summarized in the following table.

MeOH Feed Conversion of Run Rate (parts/ p-Xylene to min.) C02 (percent)the rate at which the alkanol is supplied, is a commercially importantfeature of my invention. In the previously known applications of theoxidation of alkyl benzenes by oxygen-containing gases, yield losses tocar bon dioxide were reduced by operating, when possible, at relativelylow temperatures. Although the low-temperature operation resulted in thedesired reduction in the formation of carbon dioxide, this method ofoperating was unsatisfactory in that inordinately long reaction periodswere required. My discovery that the conversion of alkyl benzenes tocarbon dioxide can be controlled by providing an alkanol in the reactionzone allows preparation of carboalkoxy benzenes at relatively hightemperatures without excessive conversion to carbon dioxide and thuseliminates some of the inadequacies of previously known methods for thepreparation of the carboalkoxy compounds.

An essential feature of the present process is the removal of water fromthe reaction zone to promote the production of esters. In the examples,the Water was removed by eliminating reflux from the condenser andleading all condensed vapors to a decanter. By conventional means,arrangements can be made for separating the water from the condensedvapors and returning the unreacted alkyl benzene and alkanol to the reaction zone.

As has been illustrated in the examples, the process is applicable notonly to such starting materials as monoalkyl-substituted benzenes, suchas toluene, ethylbenzene, and cumene, polyalkyl-substituted benzenes,such as 0-, m-, and p-xylene and durene, but also to partially oxidizedand esterified derivatives of said benzenes, such as methyl p-toluate,and mixtures of these compounds, such as p-xylene and methyl p-toluate.In general, the process is applicable to aromatic compounds having atleast one alkyl group having from 1 to 3 carbon atoms attached to thearomatic nucleus. In addition to the aforementioned compounds, suchcompounds include monoalkyl benzenes such as propylbenzene, polyalkylbenzenes such as cymene and the isomers of trimethylbenzene(hemimellitene, pseudocurnene, and mesitylene), partially oxidizedderivatives of said polyalkyl benzenes such as p-methylbenzyl alcohol,tolualdehyde, toluic acid, toluic anhydride, durylic acid,2,5-dimethylterephthalic acid, 4,6-dimethylisophthalic acid, andmethyltrimellitic acid, and esters of said partially oxidizedderivatives such as methyl durylate, the dimethyl esters of2,5-dimethylterephthalic acid and 4,6-dimethylisophthalic acid andtrimethyl methyltrimellitate. The term alkyl benzene as used hereinincludes not only monoand polyalkyl benzenes but also partially oxidizedderivatives of polyalkyl benzenes, esters and ethers of partiallyoxidized derivatives of polyalkyl benzenes, and mixtures of thesecompounds.

Lower-molecular-weight alkanols suitable for use in the present processinclude those alkanols containing 1 to 3 carbon atoms, i. e., methanol,ethanol, 1-propanol and isopropanol. The alkanol is fed at a rate suchthat the concentration of alkanol in the liquid reaction mass is atleast about 0.5% by weight or about 2.5% on a molar basis. Feeding thealkanol at a greater rate is not deleterious to the process of thepresent invention, since the operating conditions are such that anyexcess is removed rapidly from the reaction zone.

The process of the present invention normally will be carried out atelevated temperatures, for example at temperatures above 250 F. Byproper regulation of the air and alkanol feed rates, the temperature inthe reaction zone can be maintained at the desired level. The process ofthe present invention may be carried out at temperatures within therange of 250-750 F. The use of temperatures below 250 F. is feasible butimpractical because of the induction period and slow rates of reactionthereby encountered, whereas the use of temperatures above 750 F. isfeasible but i practical because of degradation of the products therebyincurred. Temperatures within the range of 300 and 600 F. have beenfound to be especially efiective and therefore are preferred.

In addition, the process of the present invention normally will becarried out at superatmospheric pressures, for example at pressuresabove atmospheres. In general, pressures within the range of 10 and 1000atmospheres may be used in effecting the present process. The use oflower pressures may cause losses of low-boiling reactants byvaporization, whereas the use of extremely high pressures requires theuse of complicated and expensive equipment. The use of pressures withinthe range of 250-1500 p.s.i.g. has been found to give very good results,and therefore pressures within the range are preferred.

The use of a catalyst is not a critical feature of the process of thepresent invention, although the presence of a catalyst promotesinitiation of the reaction. The use of cobalt naphthenate as thecatalyst has been exemplified; however, in general, if the presence of acatalyst in the reaction mass is desired, any of the oxidation catalystsconventionally used in the oxidation of alkyl benzenes may be employed.These catalysts include oxides, hydroxides, and inorganic and organicacid salts of cobalt, manganese, cerium, vanadium, lead, chromium, andiron. Of the organic acid salts of these metals, the toluates,naphthenates, and stearates are especially suitable for use in thisprocess. The catalyst preferably is soluble or partially soluble in thereaction mixture.

Although I do not wish to be limited by a theoretical discussion of thereactions involved in the process of the present invention, aninterrelationship appears to exist among the five reaction variables;temperature, pressure, air feed rate, alkanol feed rate, and time. Thisinterrelationship and the effect of this interrelationship upon thecomposition of the product mixture may be seen by reference to theexamples, especially Examples 7-11 which pertain to the production ofesters from durene. A comparison of Examples 9 and 10 indicates that ata lower air feed rate the product mixture contains a preponderance ofmonoand diesters, although the total yield of esters is approximatelythe same in both runs. A comparison of Examples 8 and 9 indicates thatat a lower alkanol feed rate the product mixture contains apreponderance of monoand diesters and a slightly larger amount of acids,although the total yield of esters is approximately the same in bothruns. A comparison of Examples 7 and 11 indicates that use of a shorterreaction period again results in the formation of larger amounts of themonoand diesters than the triand tetraesters without a substantialdecrease in the overall yield of esters. Therefore, it is evident thatthe desired composition of the reaction product may be obtained byproper regulation of these five reaction variables.

The examples have set forth the use of air as the oxygen-containing gas.However, as is evident, molecular oxygen or ozone or mixtures of thesegases with inert gaseous diluents, such as nitrogen, carbon dioxide, orthe like, may be substituted for the air.

The desired esters may be separated from the reaction mixture byconventional means. For example, for plant production of dimethylterephthalate by the present process, the dimethyl terephthalate wouldbe removed from the product by distillation or crystallization, and theremainder of the product returned with fresh pxylene to the reactor forfurther conversion to dimethyl terephthalate. The mixture in thedecanter is recovered easily by simple distillation to strip oif themethanol and stratification to remove the water from the p-xylene, boththe methanol and p-xylene being returned to the process.

Examples 14-l7 illustrate the process of the present invention ascarried out in a continuous manner. As may be seen, only slightmodification is required to operate the process in a continuous manner.

The invention has been described in detail in the foregoing. It will beapparent to those skilled in the art that many variations are possiblewithout departure from the scope of the invention. I intend, therefore,to be limited only by the following claims.

I claim:

1. A process for the Preparation of alkyl esters of benzene carboxylicacids which comprises introducing an oxygen-containing gassimultaneously with a 1-3 carbon alkanol into a reaction zone containingan alkyl benzene, said reaction zone being maintained at a temperaturebetween 250 F. and 750 F. and a pressure between 10 and 1000atmospheres, said alkanol being introduced at such rate as to maintainan alkanol concentration in the reaction zone of at least 0.5% byweight.

2. A process as claimed in claim 1, wherein an oxidation catalyst ispresent in the reaction zone.

3. A process for the preparation of dimethyl terephthalate whichcomprises introducing air simultaneously with methanol into a reactionzone containing pxylene, said reaction zone being maintained at atemperature between 300 and 600 F. and at a pressure of from 10 to 1000atmospheres, said methanol being introduced at such rate as to maintaina methanol concentration within the reaction zone of at least 0.5 byweight.

4. Process according to claim 1, benzene is toluene.

5. Process according to benzene is ethylbenzene.

6. Process according to benzene is cumene.

7. Process according to benzene is xylene.

8. Process according to benzene is durene.

9. Process according to benzene is methyl p-toluate.

References Cited in the file of this patent UNITED STATES PATENTS Agnewet a1. Oct. 2, 1951 Levine Sept. 22, 1953 FOREIGN PATENTS Great BritainMay 24, 1949 wherein the alkyl claim wherein the alkyl alkyl alkyl alkylalkyl claim wherein the claim wherein the claim wherein the claimwherein the UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNon 2,879,289 March 24, 1959 Winford B. Johnson It is hereby certifiedthat error appears in the printed specification of the above numberedpatent requiring correction and that the said Letters Patent should readas corrected below.

Column 3 line.2 below the table, for "-tet'ephthalate" readterephthalate' line 51, for "was" read me were column 5, line 14,

for "(10% yield," e d w y Signed and sealed this llth day of August1959,

S ALED) ttest: I KARL N ROBERT WATSON Attesting Oflicer Commissioner ofPatents

1. A PROCESS FOR THE PREPARATION OF ALKYL ESTERS OF BENZENE CARBOXYLIC ACIDS WHICH COMPRISES INTRODUCING AN OXYGEN-CONTAINING GAS SIMULTANEOUSLY WITH A 1-3 CARBON ALKANOL INTO A REACTION ZONE CONTAINING AN ALKYL BENZENE, SAID REACTION ZONE BEING MAINTAINED AT A TEMPERATURE BETWEEN 250* F. AND 750* F. AND A PRESSURE BETWEEN 10 AND 1000 ATMOSPHERES, SAID ALKANOL BEING INTRODUCED AT SUCH RATE AS TO MAINTAIN AN ALKANOL CONCENTRATION IN THE REACTION ZONE OF AT LEAST 0.5% BY WEIGHT. 